1. Field of the Invention
The present invention relates to an image pick-up apparatus having the vibration-proof and object tracking features useful for a TV camera, an electronic still camera, industrial image instrumentation equipment.
2. Related Background Art
Recently, image instruments such as TV cameras, electronic still cameras, or industrial image instrumentation equipment as mentioned above have been developed in which multiple functions and high performance are sought, and a great problem with this type of an image instrument is the deflection of the camera. As the deflection of a camera yields many problems in that it may not only degrade the quality of the image, but also decrease greatly the resolution, further causing a malfunction of the control system of an automatic focusing adjustment device, the correction of the camera deflection is noted as a quite important feature in photographing an image.
As the correction method for the camera deflection, there are various methods such as a method of detecting physically the vibration of the camera by means of an external sensor such as an angular velocity sensor, or a method of correcting the optical system by detecting the movement of the image corresponding to the deflection of the camera from an image signal, in which it is thought that the latter will be widely used in the future, because it can detect a photographing state from the image signal without the need of any special sensors.
Exemplary of an optical pick-up apparatus for detecting the deflection of camera from an image signal and correcting optically the deflection based on that signal is that shown in Japanese Laid-Open Patent Application No. 61-269572 entitled "Optical pick-up apparatus".
A configuration example for this apparatus is schematically shown in FIG. 2.
In the same figure, 101 is a variable apex angle prism, and 102 is a TV camera, which is image input means freely movable with respect to the camera body shown in "Optical pick-up apparatus" as previously described. 103 is a movement detection device, 106 is a controller, and 107 is an actuator (e.g., motor, voice coil, etc.).
This optical pick-up apparatus has an incident angle .theta. of an object varying with the vibration of the apparatus body. Thereby, the object image within a screen may be varied because the TV camera 102 captures an object image as the field.
A movement detection device outputs a difference between object positions on an immediately previous screen and the current screen, in which this difference signal is operated with predetermined control information via a control unit 106 so as to allow the actuator 107 to drive the variable apex angle prism 101 in a direction of compensating for the movement of the image, so that the monitor output has a stable image almost without vibration.
Exemplary of the method of detecting the vibration with a sensor is that shown in "Image deflection prevention technique for video camera" (National Technical Report Vol. 34, No. 6 December 1988). This shows that a lens-barrel freely rotates in a gimbal mount on a camera body, and the relative angle with respect to the camera body is controlled by detecting the angular velocity of the lens-barrel by means of two small gyros mounted on the lens-barrel. Therefore, the lens-barrel is always kept in a fixed direction, thereby providing a stable image without deflection.
However, in the above-mentioned conventional apparatuses, the following problems may arise on the control system.
The first problem will be described. Estimating the frequency characteristic of a vibration suppression ratio indicating how the output image should be stabilized for the deflection of camera has presented the frequency characteristics as shown in FIG. 3.
The conventional example as previously described has the characteristic rising upward to the right as indicated by 301 in the figure, while the latter case has the characteristic lowering downward to the right as indicated by 302, as easily understood from the previous document.
In general, as the vibration of a camera owing to trembling of hand or walking contains many frequency components caused by the breathing or walking of the operator, it is necessary for the correction of deflection to suppress the amplitude at that frequency. But, in the conventional examples, the vibration suppression ratio at the frequency of interest is smaller than those at other frequencies, and thereby has a poorer efficiency.
The second problem is that in general, as a sensor or movement detection apparatus has some detection errors or non-linearity, the conventional examples have an integral element for stabilizing the system or a circuit with a large time constant in the feedback circuit, and integrate the detection errors as well, thereby sometimes yielding drift (a phenomenon in which the output fluctuates even if the input is not varied).
The third problem is that when there occurs a step-like vibration, and subsequently no vibration, the optical image deflection correction means (variable apex angle prism and lens-barrel) is left away from a neutral position, and power continues to be consumed.
The fourth problem is that as optical image deflection correction means is limited in the correction angle, if the limit value is reached during the vibration-proof operation, the image that has been stabilized before may be deflected suddenly, thereby yielding an unclear image.
The fifth problem is that the amplitude and frequency of camera deflection are varied depending on the cause of the deflection (for example, there are vibrations due to breathing or walking of the camera operator, or vibrations of a vehicle such as a car).
And if seeking to obtain vibration-proof effects for all of them, an inconsistent problem arises in that the loop gain must be increased, thereby making the system unstable (an over-shoot increases and a resonance frequency occurs).
The sixth problem is that as the sensor or movement detection device has some detection errors or nonlinearity, an inconsistent problem also arises in that if the vibration-proof effects are sought in a low frequency band, the time constant of the circuit becomes large and errors are integrated, thereby causing drift.
The seventh problem is that when there occurs a step-like vibration, and subsequently no vibration, the optical image deflection correction means (variable apex angle prism and lens-barrel) is left away from a neutral position, and power continues to be consumed.
The eighth problem is that as optical image deflection correction means is limited in the correction angle, if the limit value is reached during the vibration-proof operation, the image that has been stabilized before is deflected suddenly, thereby yielding an unclear image.